"MiniLoginForm" is unusable

[GuySartorelli]

This class references self::$action_button_name - but that variable hasn't been defined. I suspect it's a holdover from a previous version and was relying on its parent class hierarchy to define it but in any case because that's not defined, the whole form just doesn't work and trying to instantiate the class results in this error:

[Emergency] Uncaught Error: Access to undeclared static property: SilverStripe\RealMe\Authenticator\MiniLoginForm::$action_button_name
GET /
Line 21 in /var/www/vendor/silverstripe/realme/src/Authenticator/MiniLoginForm.php

Note: Because the form doesn't work, I can't rebrand it as part of #105 so the styling will also need to be amended as part of this work.